1. Field of the Invention
The present invention relates to a method of oxidizing a nitride film on a conductive substrate. The method utilizes electric field to pass through and oxidize the nitride film, and it has wide applications because of its fast reacting speed and simple steps of operation.
2. Description of the Related Art
Silicon dioxide (SiO.sub.2) and silicon nitride (Si.sub.3 N.sub.4) are two essential materials for fabrication of semiconductor devices. The isolation property of the Si--SiO.sub.2 interface is better than that of the Si--Si.sub.3 N.sub.4 interface, and therefore SiO.sub.2 layers generally serve as isolation materials in the conventional process of micrometer scale.
To achieve high integration density and improve operation speed, the ultra-large-scale integrated circuits tend to scale down into the nanometer regime. In the nanometer process, the required thickness of an isolation layer is about 2.about.3 nm. Tunneling effects will occur in the isolation layer made of SiO.sub.2 under such thickness demand, and therefore the SiO.sub.2 layer is no longer a good material for isolation. On the contrary, the Si.sub.3 N.sub.4 films and SiO.sub.x N.sub.y films may become the essential materials for isolation in the nanometer process.
In addition, a series of photolithography steps and etching steps can carry out according to several defined patterns in fabricating semiconductor devices. During etching process, each of SiO.sub.2, Si.sub.3 N.sub.4, SiO.sub.x N.sub.y, or Si has different etching selectivity corresponding to the etching solvent; therefore they can be selected to serve as masks or etching targets. It is noted that forming oxidized regions in a nitride film is advantageous to define etching patterns in fabricating semiconductor devices.
At present, the scanning probe microscope such as atomic force microscope (AFM) and scanning tunneling microscope (STM) can generate a local electric field adjacent to its probe to carry out oxidizing process. However, its application is restricted to oxidize the surface regions of the semiconductors and metals, not yet applied for oxidizing the nitride films formed on a conductive substrate.